Osteoporosis is a disease characterized by reduced skeletal strength, due to low bone mass and deterioration of bone tissue, leading to enhanced fragility and increased fracture risk. Osteoporosis is responsible for over 1.5 million fractures in the United States. The majority of such fractures are of the hip, spine and wrist. Unfortunately, the bone loss of osteoporosis occurs without symptoms. In many instances, the first sign of osteoporosis is the occurrence of a fracture.
Low bone mineral density (BMD) is one of the major risk factors for osteoporosis and is readily measured. Thus, monitoring BMD has become the main method of assessing risk of osteoporosis. In addition, there are a variety of treatments available to slow or stop on-going loss of BMD. Slowing or stopping on-going loss of BMD may contribute to delaying or warding off the development of osteoporosis. Thus, assessing BMD, particularly at an early stage of BMD loss, is extremely helpful for prophylactic and interventional treatments.
Current technologies for assessing BMD are based on ultrasound, X-ray techniques or radioactivity. Each of these approaches, however, has drawbacks. Ultrasound methods are rapid, painless and do not expose individuals to potentially harmful X-rays or radioactivity. Disadvantageously, ultrasound methods are unable to assess BMD in hip or spine, which are the bones most likely to fracture. X-ray techniques, such as dual-energy X-ray absorptiometry (DEXA) and peripheral dual-energy X-ray absorptiometry (P-DEXA), while rapid and able to measure BMD in hip and spine, expose individuals to X-rays and are also expensive. Dual photon absorptiometry also can measure BMD in hip and spine, but requires the individual to be injected with a radioactive substance. Furthermore, none of these methods for measuring BMD is capable of predicting the risk of developing low BMD in individuals.
There are a number of factors known to raise an individual's risk of having low BMD. Some these include: diet low in calcium, cigarette smoking, and use of corticosteroids and excessive intake of alcohol. Osteoporosis also runs in families, indicating a genetic component. Indeed, epidemiological studies support the hypothesis that a large part of the variation in BMD is caused by genetic susceptibility factors.
Linkage analyses and other research have further supported this hypothesis, and have revealed that there appear to be many genes involved in susceptibility. For instance, in a first-stage autosomal genome screen, Devoto et al. (1998, Eur. J. Hum. Genet. 6:151-157) identified regions on three different chromosomes, 1p, 2p and 4q, that appear linked to spine and hip BMD. Each of these regions contains many candidate genes. Subsequent work by this group further narrowed the region on 1p to 1p36.2-1p36.3 (Devoto et al, 2001, Hum. Mol. Genet. 10:2447-2452). While narrowed, this region still encompasses many genes. Bivariate variance component linkage analysis provides additional support for the linkage of 1p36 to BMD (Devoto et al, 2005, Eur. J. Hum. Genet. 13:781-788). Linkage studies by Wilson et al. (2002, Am. J. Hum. Genet. 72:114-155) provide independent support for the association of 1p36 with BMD and also identify 3p21 as another region linked to BMD.
Polymorphisms are allelic variants that occur in a population. A single nucleotide polymorphism (SNP) is a position in a particular DNA sequence characterized by the presence in a population of two, three or four different nucleotides at that position. The most common SNPs have two different nucleotides and are thus biallelic. Identification of SNPs associated with disease susceptibility is invaluable for screening and early initiation of prophylactic treatments. Furthermore, SNP identification may eventually lead to identifying genes that contribute to the development of low BMD and osteoporosis, which could aid the development of new therapeutic agents and treatment strategies.
There are several examples of specific SNPs that are correlated with elevated risk of low BMD. For instance, in US patent publication 2003/0235847, methods for determining risk of low BMD by assessing the nucleotide identity of one or more polymorphisms in the sclerostin gene on chromosome 17 are disclosed. Similarly, US patent publication number 2003/0176344 discloses methods for determining susceptibility to osteoporosis by assessing the nucleotide identity of one or more polymorphisms in the BMP2 gene on chromosome 20. U.S. Pat. No. 5,998,137 discloses methods for determining susceptibility to osteoporosis by assessing the nucleotide identity of one or more polymorphisms in the promoter of the TGF beta 1 gene. U.S. Pat. No. 6,825,336 discloses methods of determining susceptibility to osteoporosis by assessing the nucleotide identity of one or more single nucleotide polymorphisms in genes already known to be associated with osteoporosis. Reneland et al. (2005, BMC Medical Genetics 6:9) teach an association between specific SNP variants in the phosphodiesterase 4D locus on chromosome 5 and low BMD. U.S. Pat. No. 6,762,023 discloses methods for determining susceptibility to osteoporosis by assessing the nucleotide identity of one or more polymorphisms in the TNF alpha 2 receptor gene on chromosome 1.
There is, however, no evidence that the SNPs identified to date as associated with low BMD and/or osteoporosis are sufficient to identify all individuals at risk. Given that multiple genes appear to be involved in low BMD susceptibility, there is an on-going need in the art for identifying SNPs that are linked to low BMD. Furthermore, there is need in the art for a method of assessing risk of low BMD and therefore of elevated risk of osteoporosis, which does not require patient exposure to X-rays or radioactivity. This invention meets these needs.